The present invention relates to a molding material which can be used for injection, extrusion and compression moldings and provide a molded product having a better distribution of fibers with fewer fibers being broken, and with higher mechanical strength.
According to conventionally well-known methods of producing a composite of thermoplastic resin reinforced with fibers, glass fibers, each about 3 mm long, for example, are dry-blended with the thermoplastic resin and mixed and granulated into pellets by means of the extruder.
In these conventional methods, however, each of the glass fibers tend to come loose into filaments which intertwine and cling to one another when the dry-blended material is mixed by the extruder. As a result, the glass fibers are not distributed uniformly in the pellet. Further, the glass fibers are broken and the glass fibers of various lengths are thus contained non-uniformly in the pellet, with their median held about 0.3 mm. These factors reduce the reinforcing effect of the thermoplastic resin with glass fibers.
Although the upper limit of the glass fibers contained or mixed is conventionally 30 weight %, a molding material in which the glass fibers are contained to a higher extent has been used in an attempt to enhance mechanical strength. When the weight percentage of the glass fibers is higher than 30, however, it is difficult to uniformly distribute the glass fibers in the pellets during the mixing process. Therefore, such molding material containing the glass fibers at a value higher than 30 weight % cannot be provided.
In order to solve the above-mentioned drawbacks, there have been proposed some methods of coating the glass fibers with thermoplastic resin. According to the method disclosed by Japanese Patent Publication Sho 49-41105, for example, molten thermoplastic resin is introduced into openings of the die, while passing bundles of glass fibers through the openings, by the extruder to coat the bundles of glass fibers with the thermoplastic resin, and the bundles of glass fibers thus coated are cooled and then cut by a certain length to provide cylindrical pellets of a molding material. With this molding material however, the fibers in the pellets are the same length as the pellets, keeping them long, before the pellets are molded, but as the fibers are contained to a higher extent, they cannot be distributed uniformly in the molded product. Further, most of the fibers contained are broken because they are shear-stressed while being passed between the barrel and the screw at the supply zone in the extruder, and the average length of the fibers in the molded product becomes about 0.5 mm. This prevents the fibers from providing their reinforcing effect to the desired extent.